Canberra, April 7 (IANS): A former official of Australia's Labour Party, who is believed to have fought for Kurdish forces against the Islamic State (IS), should be investigated, leader of opposition said on Tuesday.

Former president of Northern Territory's Labour Party, Matthew Gardiner was detained for questioning upon his arrival on Sunday at the Darwin airport after spending several months abroad where he was suspected of fighting with Kurdish forces in the Middle East, Xinhua news agency reported.

Leader of Opposition Bill Shorten said Australians who visited the prohibited area of Syria and Iraq should be thoroughly investigated.

Shorten said the Australian federal police should investigate anyone who traveled to the zones that parliament last year declared out of bounds for Australian citizens.

"We say to any Australian who thinks they should take part in these conflict zones, don't do it. And if you return, the police will appropriately investigate the matter consistent with the law," he said.

According to official reports, the Kurds, who Gardiner was believed to have helped, were involved in a battle with IS since the jihadi group invaded their territory last year.

There are an estimated 100 Westerners currently fighting with Kurdish forces. In February, Australian Army reservist Ashley Johnston, 28, was killed on the frontline in Syria, fighting with Kurdish forces against IS militants.